1. Field of the Invention
This invention relates to a camera which permits a change-over of focal length of the photo taking optical system thereof.
2. Description of the Prior Art
During recent years, there have been proposed cameras of the kind having a focal length varying device which permits photographing at various focal lengths such as a normal focal length, a telephoto focal length, etc. In a camera of this kind, however, the F-number of the aperture and the depth of field of a photo taking optical system will vary as a result of a focal length change-over. Therefore, the arrangement of the prior art to automatically focus the photo taking optical system by means of the automatic focusing device has presented various problems. For example, it has been known, from U.S. Pat. No. 4,183,643, etc., to set a photo taking optical system in a certain predetermined position where the widest in-focus range is obtainable when a distance to an object to be photographed is undetectable by means of a distance measuring circuit in an automatic focusing type camera. This predetermined position is generally a hyper-focal point in which the optical system is focused on a hyper-focal distance with the farthest point within the depth of field becoming an infinity distance (hereinafter the predetermined position will be called a fix point). However, the distance from the photo taking optical system to the hyper-focal point is in direct proportion to the square of the focal length of the photo taking optical system and in inverse proportion to the square of the aperture F-number thereof. In the case of a variable focal length type camera having a photo taking optical system which is shiftable, for example, for a normal optical system of focal length 40 mm (F 2.8) to a telephoto optical system of focal length 70 mm (F 5.6), the hyper-focal point varies to a great extent according to the focal length change-over. Therefore, in case such a variable focal length camera is provided with an automatic focusing device, the probability of an out-of-focus photographing operation increases under a condition where distance cannot be measured if the above-stated fix point is left unvarying despite a focal length change-over.
Further, in the case of an automatic focusing type camera provided with a flash device, problems such as under exposure, etc. tend to arise, even when a focusing operation is appositely carried out, because the flash light quantity of the flash device is limited. To be more specific, between a guide number GN which denotes the flash light quantity to be emitted from the flash device, an F-number of the aperture of the photo taking optical system and a distance "d" between the flash device and an object to be photographed, there is the following relation: GN=F.times.d. With the aperture F-number unvarying, a light quantity larger than the guide number of the flash device theoretically becomes necessary when the object is located at a long distance. To solve this problem, there have been proposed, as known from Japanese Utility Model Application Laid-Open No. Sho 54-18229, etc., an arrangement wherein the shift of a photo taking optical system toward an infinity position is limited to a distance within a flash photographable distance range determined jointly by the F-number of the aperture and the guide number of the flash device (hereinafter this distance will be called flash device far point) even when the object is located at a long distance. Thus, the camera is restricted to photographing within a range which can be sufficiently covered with the flash light quantity of the flash device.
However, if a camera of this kind is provided with the focal length varying device mentioned in the foregoing, the above-stated far point of the flash device will vary accordingly as the aperture F-number varies, because the F-number varies every time the focal length of the photo taking optical system is shifted. Thus, a camera of this kind also presents a problem.